fileset: insert hints where status should be computed This will allow us to compute status against a narrowed set of files. For example, "path:build/ & (unknown() + missing())" is rewritten as "path:build/ & <withstatus>(unknown() + missing(), 'unknown missing')", and the status call can be narrowed by the left-hand-side matcher, "path:build/". mctx.buildstatus() calls will be solely processed by getmatchwithstatus().